5 Ways to Control Humidity with Small HVAC Systems
Owners of retail shopping centers and other commercial buildings want long lasting, inexpensive HVAC equipment that can be on site in three or four weeks and can be maintained by a local service contractor. Enter the constant volume direct expansion (DX) air conditioning system, most commonly the small packaged rooftop unit and its partner in crime, the split system. Unfortunately, these systems won’t control indoor humidity in summer months unless your mechanical engineers take important precautions during design. If you lose humidity control in your store or office, odors develop, turning off customers, and everyone feels sticky even if it is cool inside. We have five steps to prevent this.
What’s the problem?
In a nutshell, an air conditioning system controlled only by a room thermostat does not directly control humidity. Dehumidification occurs only when the cooling coil is cold enough to condense water from the airstream, and this only happens when the thermostat is calling for cooling. The trouble starts once the temperature in the occupied zone reaches the thermostat setpoint and the cooling cycles off. The supply fan runs continuously to provide ventilation, circulating room air mixed with warm, humid outside air back to the occupied space, and picking up moisture that was left on the face of the cooling coil at the end of the last cooling cycle. As a result, the indoor relative humidity rises until the thermostat calls for cooling again, and if subsequent cooling cycles are too short to properly dehumidify the space, we lose humidity control.
What’s the solution?
Fortunately, there are ways that we can improve indoor humidity levels without breaking the bank, and here are five possibilities. And as an added bonus, most of these suggestions lower energy bills.
1) Right-size the cooling equipment. Oversized equipment will cool the space too quickly and exacerbate the humidity problem. Tell your retail MEP engineer to avoid the temptation to oversize equipment just to cover that one day a year that might hit 100 degrees, and throw safety factors out the window.
Beware of direct replacement scenarios where the new equipment capacity is selected to match existing. Even if the old equipment wasn’t oversized in 1990, it will be today thanks to the new roof and windows, and all that modern lighting and office equipment.
2) Lower the system airflow. Most constant volume DX systems can comfortably operate at 350 cfm per ton, and there are few modern office/retail applications in humid climates that need airflow rates higher than this. Take advantage of it. The lower the system airflow, the longer the system run time until the thermostat is satisfied, and the more moisture that is removed from the air. What’s more, this adjustment can be applied to existing systems.
3) Use variable capacity equipment. Single zone variable air volume (SZ-VAV) systems reduce supply fan airflow at part load conditions. A lower fan speed matched to the first stage of cooling keeps that cooling coil and supply air cold, enhancing moisture removal at part load. SZ-VAV is required by ASHRAE 90.1-2010 for systems 10 tons and up, and many manufacturers offer it as an option for smaller equipment as well.
Variable Refrigerant Flow (VRF) systems are also gaining popularity. These systems modulate compressor speed as well as supply fan speed to further reduce compressor cycling, and so provide good humidity control.
4) Incorporate Demand Controlled Ventilation (DCV). The outside air requirement for a retail tenant space can account for two thirds or more of the humidity load at peak design conditions. Why bring all that humidity into the building when it is not required? Code allows the ventilation rate to be dynamically reset based on actual occupants present, so take advantage of it. For an open retail space, a CO2 sensor installed in the return air section of the rooftop unit is a simple option, and this can be retrofitted to existing equipment in many cases.
5) Add Direct Dehumidification Control. The first four suggestions above indirectly control humidity, either by improving the system’s inherent ability to remove moisture, or in the case of DCV by reducing the amount of moisture that the system needs to remove in the first place. By contrast, direct dehumidification allows the equipment to be controlled by a humidity sensor, so that desired space humidity levels can be dialed in, as well as space temperature.
Most HVAC equipment manufacturers offer a factory installed direct dehumidification equipment option for their packaged rooftop equipment, commonly referred to as a Hot Gas Reheat system. This option may be best suited for high humidity applications such as restaurant dining rooms or assembly areas.
With properly sized equipment and appropriate selection of options and accessories, the venerable small DX system can provide acceptable humidity control in many comfort cooling applications, such as retail stores and small offices.